1. Field of the Invention
The present invention relates to surgical instruments for applying rows of fasteners to body tissue, and more particularly to anvil members employed in such instruments.
2. Background of the Art.
Surgical fastener applying instruments used for applying parallel rows of fasteners to body tissue are well known in the art and are commonly used for transecting or reconnecting body tissue such as intestinal, gastric, or lung tissue. The fasteners can be in the form of staples which are closed by crimping the staple legs, or two part fasteners which are closed by locking together a fastener portion with a retainer portion.
Surgical staple applying instruments are illustrated and described, for example, in U.S. Pat. Nos. 4,633,861; 4,633,874; 4,863,088; 3,079,606; 3,490,675; 3,499,591; and 4,429,695. Generally, such staplers include a forked distal end. The body tissue to be operated upon is held securely between two forks of the instrument and the staples are sequentially applied to the tissue.
One fork comprises staple holding and pushing means. Usually, at least two parallel rows of staples are held in slots oriented transversely to the longitudinal axis of the instrument. Pusher elements are located behind the staples. When a camming bar is moved longitudinally by a drive member the pusher elements are driven through the slots, thereby pushing the staples out into the body tissue. The other fork comprises an anvil, i.e., staple closing means. The legs of the staple penetrate the body tissue and enter staple closing depressions in the anvil where they are crimped.
The surgical fasteners must be properly closed to insure that the body tissue is securely fastened, otherwise bleeding or other complications can occur from poor hemostasis of the fastened tissue. Proper closure of the fasteners requires precise alignment of the two forks of the instrument so that the fasteners are precisely aligned with the closing means. However, bending or deflection of the anvil can occur when the two forks are clamped together with body tissue in between, and this bending can cause misalignment of the staples and the staple closing depressions. Deflection can also occur as a result of vertical forces applied to the anvil during contact by the fasteners. More deflection occurs at the distal end of the anvil. The longer the anvil the greater is the tendency to bend. Hence, one way of dealing with bending is to limit the length of the anvil. But in many applications it is desirable to use a longer anvil, which would be able to lay down longer rows of staples.
Another way of reducing bending is to make the anvil stronger. Anvils are presently fabricated by cold working sheet stainless steel of about 0.05 inches to about 0.075 inches in thickness into the desired shape. Stronger anvils can be made by increasing the thickness of the steel. However, this presents disadvantages insofar as the weight and cost of the anvil, as well as the difficulty of working thicker steel, are increased. Using an alternative, harder material of construction can increase the rigidity of the anvil, but possibly at the expense of sacrificing desirable properties of the stainless steel. Therefore, the need exists to fabricate an anvil which is stronger and more resistant to bending while avoiding the disadvantages just mentioned.